Shoulder dystocia (SD) refers to excessive difficulty in delivering the baby vaginally after the head is born because of the position or diameter of the shoulders. The reported incidence of shoulder dystocia varies from 0.6% to 1.4% among vaginal deliveries. When the baby's head is born and the shoulders are trapped within the birth canal, the baby's chest is compressed within the birth canal preventing it from breathing and the umbilical cord is compressed reducing the amount of oxygen supplied to the baby. Special delivery procedures are recommended to ease the passage of the shoulders and allow the baby to be completely born without undue delay or force. During such a birth, the nerves in the baby's neck, leading to the arm (the brachial plexus), can be stretched and injured. Neonatal injuries associated with shoulder dystocia include brachial plexus injury in 4 to 40%, clavicular fracture in 5.1 to 7.5% and very rarely brain damage or death. Although most brachial plexus injuries will recover, those that do not may result in permanent weakness or paralysis in the arm. Reported rates of persistent palsy range from 3 to 50% (ACOG Technical Bulletin Number 40, 2002, Baskett T F, 2002, Gherman R B. 2002). Injury from shoulder dystocia is a common cause of litigation and failure to adequately predict and or communicate the risk of shoulder dystocia and injury is commonly alleged in legal actions.
If the likelihood of shoulder dystocia with neonatal injury for a given obstetrics patient could be accurately estimated, it would desirable to intervene when the likelihood of shoulder dystocia is high such as to avoid (or at least reduce the rate of) neonatal injuries caused by this situation. Intervention may be in the form of elective cesarean delivery for example.
For the reader's information, the following are a few studies related to shoulder dystocia:                1. ACOG practice bulletin: Shoulder dystocia. Number 40, November 2002. (Replaces practice pattern number 7, October 1997).        2. Baskett T F. Shoulder Dystocia. Best Practice & Research Clinical Obstetrics and Gynecology 2002;16:57-68.        3. Rouise D J, Owen J, Goldenberg R L, Cliver S P. The Effectiveness and Costs of Elective Cesarean Delivery for Fetal Macrosomia diagnosed by Ultrasound. JAMA. 11996;276(18): 480-6.        4. Gherman R B. Shoulder Dystocia: An Evidence-Based Evaluation of the Obstetric Nightmare. Clinical Obstetrics and Gynecology 2002;45: 345-362.The contents of the above documents are herein incorporated by reference.        
Generally, professional societies recommend intervention based on a hard threshold of estimated fetal weight, which may be modified by the presence or absence of diabetes, or recommend making a qualitative judgment about risk factors in general. Large fetal weight is generally considered as being the most consistent and important factor in predicting an increase in the risk of shoulder dystocia (SD). For that reason, some professional societies recommend elective cesarean if the fetal weight is estimated to be over 5000 g in women without diabetes and 4500 g in women with diabetes. Other studies could not justify intervention at the 4500 gram estimated weight threshold even in the presence of gestational diabetes.
A deficiency associated with existing methods, such as the ones described in the above noted publications, is that in order to detect a high percentage of babies with the relatively rare condition of shoulder dystocia, an unacceptable rate of false positive predictions occurs in the more common uncomplicated pregnancies. This could lead to excessive numbers of unnecessary interventions in women rendering the prediction of little clinical value. For example, it has been estimated that by having a policy of intervention at 4500 gram, an extra 443 cesareans would be required to avoid a single case of permanent brachial palsy injury. The financial cost of such a policy would be about $930,000US for each avoided case of permanent brachial palsy injury without counting the emotional and physical trauma of the 443 mothers who underwent essentially unnecessary cesareans. (Rouse D J et al 1996—reference 3 above) Moreover, several studies have shown that a substantial number of cases of shoulder dystocia occurs in women with infants weighing less than 4000 g. Therefore, even though fetal weight is an important factor in predicting shoulder dystocia, it is nevertheless not sufficiently reliable and does not provide a suitable prediction of the level of risk shoulder dystocia for an individual mother. In fact, the ACOG Practice bulletin (reference 1 above) concludes, “Shoulder dystocia is most often unpredictable and unpreventable”.
Therefore, in the context of the above, there is a need to provide a method and apparatus for estimating a level of risk of shoulder dystocia associated to an obstetrics patient that alleviates at least in part problems associated with the existing methods and devices.